Cellulose bonded nonwoven fiber fabric and method for the production thereof

ABSTRACT

The invention relates to a bonded nonwoven fiber fabric and packaging films strengthened with such a fabric, especially sausage casings based on celluloses. The nonwoven fiber fabric itself can be used as teabag paper. Bonding is achieved through treatment of a solution comprising cellulose, N-methylmorpholime-N-oxide and water. Bonding can be strengthened by a polyamine polyamide epichlorohydrin resin added to the fibrous pulp.

This application claims benefit of priority under 371 forPCT/EP97/04941, filed Sep. 10, 1997.

The invention relates to a cellulose-bonded nonwoven fiber fabric and aprocess for the production thereof. It further relates to the use ofthis nonwoven fiber fabric for the production of food packaging, inparticular teabags or sausage casings.

Fiber-reinforced seamless cellulose casings are usually produced by theviscose process. In this process, a nonwoven fiber fabric, for examplemade of hemp fiber paper, which has been formed into a tube is coated onthe inside and/or outside with an alkaline viscose solution. Thematerial coated on the inside, outside or both with viscose thusproduced is then treated with an acidic coagulation liquid whichprecipitates the cellulose xanthogenate and regenerates it to formcellulose hydrate. Generally, enough viscose solution is applied tocover the nonwoven fiber fabric completely on one or both sides with alayer of regenerated cellulose. Fiber-reinforced cellulose casings arevery widely used as sausage casings.

It is also known to coat with viscose flat webs made of a nonwoven fiberfabric. In this process, the viscose-coated material is coagulated inthe manner described and regenerated. The fiber-reinforced celluloseflat films may likewise be processed into tubular casings if they areappropriately bent and the edges sewed together, glued or sealed. Suchcasings are also suitable as sausage casings.

The nonwoven fiber fabric is produced in a usual manner from cellulosefibers. To increase its strength, it is generally bonded. Thus U.S. Pat.No. 3,135,613 discloses the production of a wet-strength hemp fiberpaper. The increased strength is achieved by treatment with a dilutealkaline viscose solution, drying and regeneration of the celluloseusing dilute sulfuric acid. The paper is then washed until it isacid-free and is finally dried. The regenerated cellulose coatingserving as bonding is so thin that the porous structure of the paper isretained.

However, a pure cellulose bonding is not sufficiently alkali-resistantand stable to hydrolysis. During a subsequent coating of the paper withalkaline viscose solution, the existing cellulose again partiallydissolves and the fibers loosen. Tubular films having a fiberreinforcement of this type therefore have a tendency to burst, even at alow internal pressure.

To avoid this disadvantage, nonwoven fiber fabrics have also been usedwhich are set solely with synthetic resins. According to GB-A 1 091 105,the cellulose fibers are bonded with an alkali-curable resin, forexample a polyethyleneimine resin or an epichlorohydrin-crosslinkedpolyamide. In U.S. Pat. No. 3,484,256, for this purpose use is made of amixture of a cationic heat-curable resin and a polyacrylamide. Cellulosecasings reinforced with this material do not guarantee the burstingstrength which is demanded for certain sausage types.

Resin bonds are generally not sufficiently heat stable. The viscosesolution can in addition not penetrate resin-bonded nonwoven fiberfabrics sufficiently. Furthermore, the resin causes the regeneratedcellulose to adhere insufficiently to the fibers.

The object of the invention is to provide in an environmentallycompatible manner a nonwoven fiber fabric which has wet strength, isalkali- and hydrolysis-resistant, is optimally penetrated by the viscoseand bonds firmly to regenerated cellulose hydrate. In particular, thetubular food casings produced therefrom by viscose-coating are to havean optimum strength, extensibility and swellability, as well as a goodshrinkage behavior.

This object is achieved by a nonwoven fiber fabric whose fibers werebonded by

a) treatment with a solution of cellulose in a mixture ofN-methylmorpholine N-oxide (NMMO) and water,

b) precipitation of the cellulose using an aqueous NMMO solution,

c) washing to remove the NMMO and, if appropriate,

d) drying.

The nonwoven fiber fabric can, as is customary, be produced by running ascreen through a fiber pulp and then partially or completely drying theresultant nonwoven fiber fabric. The fiber pulp preferably comprisesfrom about 0.1 to 2% by weight of cellulose fibers, hemp fibers beingpreferred. Other constituents, such as resins or sizes, can also befurther added to the fiber pulp. The nonwoven fiber fabric can also beimpregnated or coated, preferably likewise with resins or sizes.

In a particularly preferred embodiment, apolyamine-polyamide-epichlorohydrin resin (also calledpolyamidoamine-epichlorohydrin resin), a urea-formaldehyde resin, amelamine-formaldehyde resin or a urea-melamine-formaldehyde resin isadded to the fiber pulp. The amount of this resin is preferably from 0.5to 2% by weight, based on the total weight of the fiber pulp. The resinmigrates during drying to the points of intersection of the fibers andreacts during curing with the fibers and also with itself. This furtherincreases the stability.

The nonwoven fiber fabric according to the invention comprises fromabout 0.5 to 9% by weight, preferably from 1 to 3% by weight,particularly preferably from 1.5 to 2% by weight, of cellulose based onits total weight. It is already usable as such, e.g. to produce teabagstherefrom. It then preferably has a weight of from about 8 to 20 g/m².However, particularly advantageously, it may be viscose-coated andprocessed to form fiber-reinforced food casings. It then preferably hasa weight of from 12 to 30 g/m², particularly preferably from 15 to 28g/m².

The invention thus also relates to a fiber-reinforced food casing orpackaging film produced by the viscose process, wherein the fiberreinforcement consists of the nonwoven fiber fabric according to theinvention. It is principally used as sausage casing.

The invention also includes a process for producing the nonwoven fiberfabric according to the invention, which comprises the following stagesin the sequence specified:

providing a suspension which comprises cellulose fibers and preferably,in addition, at least one polyamine-polyamide-epichlorohydrin resin,

forming a nonwoven fiber fabric from the suspension (e.g. by filteringoff through an inclined screen),

partial or complete drying of the nonwoven fabric,

applying a solution of cellulose in NMMO/water to the nonwoven fabric,

precipitating the cellulose using an aqueous NMMO solution,

washing the nonwoven fabric (generally with water) until the nonwovenfabric is virtually free of NMMO and

drying until the desired degree of residual moisture is reached.

Treatment with the cellulose/NMMO/water solution preferably takes placeby immersion into a corresponding bath.

The cellulose/NMMO/water solution preferably comprises from about 0.5 to2.0% by weight of cellulose, based on its total weight. The NMMO/watermixture used as solvent preferably consists of from 85 to 90% by weightof NMMO and from 15 to 10% by weight of water. The preparation ofcellulose/NMMO/water solutions is also described in DE-A 196 07 953.

The aqueous NMMO solution used for the precipitation preferablycomprises from about 5 to 15% by weight of NMMO, particularly preferablyabout 10% by weight of NMMO, based on its total weight.

The nonwoven fiber fabric according to the invention optimally complieswith the objects set. In particular, it shows a higher wet strength anddry strength. In addition, it has the desired porous surface structure.The cellulose applied from NMMO solution by the process according to theinvention is more compact, denser and more crystalline (from 55 to 65%crystalline fraction) than that applied by the viscose process (from 34to 42% crystalline fraction). It is therefore markedly less attackedduring a subsequent coating by viscose solution or other alkalinesolutions. This is another advantage of the nonwoven fiber fabricaccording to the invention.

At least as important as the higher crystallinity is the fact that thenonwoven fiber fabrics according to the invention may be producedsimply. In contrast to the viscose process, no exhaust air or wastewater problems occur during production. The NMMO is approximately 99.5%recovered in the process.

To produce a packaging film or food casing, webs of the nonwoven fiberfabric according to the invention are viscose-coated on one or bothsides. Seamless tubular films or casings may be obtained by bending thewebs of the nonwoven fiber fabric to form a tube and impregnating andcoating them on the outside and/or inside with a customary alkalineviscose solution. The viscose-coated surface is then treated with anacidic spinning liquid which is customary for viscose precipitation andusually comprises sulfuric acid. Spinning liquid can be in a baththrough which the viscose-coated, if appropriate tubular, fiber webruns, or using a nozzle, it is applied to the viscose-coated fiber web.After running through the regeneration and wash baths customary in theproduction of films from cellulose hydrate, the fiber-reinforcedcellulose film is dried. In the end product, the nonwoven fiber fabricis covered on one or both sides with a cellulose layer so that itssurface structure is no longer visible.

It the tubular food casing according to the invention is to be used as asausage casing, it can be further provided on the inside and/or outsidewith the coatings or impregnations customary therefor. For example, onthe inside and/or the outside a barrier layer against oxygen or watervapor can be applied. Depending on the type of the sausage emulsion, aninner coating to improve the peeling behavior or to improve the adhesionbetween emulsion and casing can also bring advantages. In certain cases,finally, a fungicidal coating on the outside is appropriate. To obtaincolored sausage casings, in addition, color pigments, e.g. carbon blackor TiO₂, can be present in the cellulose layer. The tubular sausagecasing can advantageously be traded in the form of "shirred sticks"(shirred sections, one end of which can already be tied) or flattened orrolled up as "spooled goods".

The invention is described in more detail by the following examples. Allpercentages are percentages by weight, unless stated otherwise.

EXAMPLE 1

Hemp fibers were laid by a conventional process from a paper pulp, inwhich the cellulose fiber content was from 0.1 to 0.2%, on an inclinedscreen to form a coarse-structured fiber paper of 21 g/m². The paper wasconducted over heated large-diameter rollers and dried. The dry paperweb was then conducted through a vat which contained a 0.8% strengthcellulose solution in an 87.7% strength aqueous NMMO solution at atemperature of 95° C. The paper was squeezed slightly and run through asecond vat which contained a 15% strength NMMO solution at a temperatureof 20° C. The residual NMMO was extracted with pure water in a thirdvat. Thereafter, the paper was dried again and wound up. The cellulosecontent of the nonwoven fiber fabric was approximately 1.2%. In the wetstate it had a rupture strength (mean of the longitudinal and transverserupture strength) of from 6.5 to 7 N/mm² and a rupture strain (mean oflongitudinal and transverse elongation at break) of from 7 to 8%, basedon the initial length. With a 10 minute treatment in 6% strength sodiumhydroxide solution, the nonwoven fiber fabric lost only from 15 to 20%of its strength, and the elongation at break remained unchanged. In theimpregnation of the nonwoven fiber fabric with alkaline viscosesolution, perfect viscose penetration took place, and the precipitatedcellulose hydrate adheres to the fibers very well.

A tube (caliber 75 mm) coated with viscose on the outside having thisnonwoven fabric as a reinforcing inner layer achieved a burstingpressure (wet) of 79 kPa, i.e. 15.5% above the customary nominal value;the static extension at 21 kPa was 82.5 mm (permitted range: 80.3 to83.3 mm). Mechanically shirred tubes could be processed on automaticstuffing machines. The casings were more stable than the standardcasings provided with the inner fiber layer conventional hitherto.

EXAMPLE 2

An amount (0.8%) of still water-solublepolyamine-polyamide-epichlorohydrin resin was added to the hemp fiberpaper pulp so that the resin content of the dried paper reachedapproximately 2%. The nonwoven fiber fabric was then produced from thisand dried, as described in Example 1. It had a weight of 23.7 g/m². Itwas run firstly through a 95° C. solution of cellulose in 87.7% strengthNMMO, then through a 10% strength NMMO solution at 20° C. and thenthrough a water vat, dried again and wound up. In the wet state, therupture strength was from 8 to 9 N/mm² (mean of longitudinal+transverse)and the rupture strain was from 6 to 6.5%. On alkali treatment, thisnonwoven fabric lost only from 12 to 15% of its wet strength, and therupture strain was unchanged. The nonwoven fiber fabric was shaped toform a tube and impregnated on its outside with alkaline viscosesolution. The 90 mm caliber tube obtained after acid precipitation andcustomary regeneration reached a bursting pressure of 72 kPa, i.e. 17%above the value usually required. The static extension at 21 kPa was100.5 mm (required range: from 99 to 102 mm). The casings wereextraordinarily stable, could be shirred without problem and could bestuffed with sausage mixture on automatic stuffing machines. Thestuffing, shrinkage, ripening and peeling behavior were normal.

EXAMPLE 3

Similarly to Example 1, a hemp fiber paper having a weight per unit areaof 25.4 g/m² was produced. It was then run through a 1.2% strengthcellulose solution in 87.7% strength aqueous NMMO at 90° C., thenprecipitated in a 12% strength NMMO solution, washed in a further washvat, then dried and wound up. The paper, in the wet state, showed arupture strength of 9 N/mm² and a rupture strain of 7%.

A 120 mm caliber tube viscose-coated on the outside showed, after thecustomary precipitation and regeneration, a bursting pressure of 64 kPa,or 18.5% above the required value, and the static extension was 135 mm,at 21 kPa (required range: from 133 to 137 mm). It could be processedwithout defect.

We claim:
 1. A nonwoven fiber fabric whose fibers were bonded bya)treatment with a solution of cellulose in a mixture ofN-methylmorpholine N-oxide (NMMO) and water, b) precipitation of thecellulose using an aqueous NMMO solution, c) washing to remove the NMMOand, if appropriate, d) drying.
 2. A nonwoven fiber fabric as claimed inclaim 1, wherein the cellulose content is from 0.5 to 5% by weight basedon its total weight.
 3. A nonwoven fiber fabric as claimed in claim 1,wherein the fibers are hemp fibers.
 4. A process for producing a bondednonwoven fiber fabric having the following stages in the sequencespecified:providing a suspension which comprises cellulose fibers,forming a nonwoven fiber fabric from the suspension, partial or completedrying of the nonwoven fabric and bonding of the nonwoven fabric,whichcomprises, for the purpose of bonding, applying to the nonwoven fabric asolution of cellulose in an NMMO/water mixture, precipitating thecellulose using an aqueous NMMO solution, washing the nonwoven fabricvirtually free from NMMO and drying it.
 5. The process as claimed inclaim 4, wherein the suspension additionally comprises at least onepolyamine-polyamide-epichlorohydrin resin, a urea-formaldehyde resin ora melamine-formaldehyde resin.
 6. The process as claimed in claim 4,wherein the solution is applied by immersing the nonwoven fabric into abath containing a cellulose/NMMO/water solution.
 7. Teabags comprisingthe nonwoven fiber fabric as claimed in claim
 1. 8. A fiber-reinforcedfood casing or packaging film produced by the viscose process, whereinthe fiber reinforcement consists of a nonwoven fiber fabric as claimedin claim
 1. 9. The process for producing the food casing or packagingfilm as claimed in claim 8, in which a nonwoven fiber fabric isviscose-coated on one or both sides, wherein the nonwoven fiber fabriccomprises a nonwoven fiber fabric whose fibers were bonded bya)treatment with a solution of cellulose in a mixture ofN-methylmorpholine N-oxide (NMMO) and water, b) precipitation of thecellulose using an aqueous NMMO solution, c) wasing to remove the NMMOand, if appropriate, d) drying.
 10. The process as claimed in claim 9,wherein the nonwoven fiber fabric was bent to form a tube prior to theviscose-coating.
 11. A nonwoven fiber fabric as claimed in claim 1,wherein the cellulose content is from 1 to 3% by weight based on itstotal weight.
 12. A nonwoven fiber fabric as claimed in claim 1, whereinthe cellulose content is from 1.5 to 2% by weight based on its totalweight.
 13. Reinforced packaging films comprising the nonwoven fiberfabric as claimed in claim
 1. 14. Tubular food casings comprising thenonwoven fiber fabric as claimed in claim 1.